The subject invention generally relates to ink jet printing, and more particularly to a thin film ink jet printhead having FET drive circuits configured to compensate for parasitic power dissipation along a ground bus.
The art of ink jet printing is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with ink jet technology for producing printed media. The contributions of Hewlett-Packard Company to ink jet technology are described, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985); Vol. 39, No. 5 (October 1988); Vol. 43, No. 4 (August 1992); Vol. 43, No. 6 (December 1992); and Vol. 45, No. 1 (February 1994); all incorporated herein by reference.
Generally, an ink jet image is formed pursuant to precise placement on a print medium of ink drops emitted by an ink drop generating device known as an ink jet printhead. Typically, an ink jet printhead is supported on a movable print carriage that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed.
A typical Hewlett-Packard ink jet printhead includes an array of precisely formed nozzles in an orifice plate that is attached to an ink barrier layer which in turn is attached to a thin film substructure that implements ink firing heater resistors and apparatus for enabling the resistors. The ink barrier layer defines ink channels including ink chambers disposed over associated ink firing resistors, and the nozzles in the orifice plate are aligned with associated ink chambers. Ink drop generator regions are formed by the ink chambers and portions of the thin film substructure and the orifice plate that are adjacent the ink chambers.
The thin film substructure is typically comprised of a substrate such as silicon on which are formed various thin film layers that form thin film ink firing resistors, apparatus for enabling the resistors, and also inter-connections to bonding pads that are provided for external electrical connections to the printhead. The ink barrier layer is typically a polymer material that is laminated as a dry film to the thin film substructure, and is designed to be photodefinable and both UV and thermally curable. In an ink jet printhead of a slot feed design, ink is fed from one or more ink reservoirs to the various ink chambers through one or more ink feed slots formed in the substrate.
An example of the physical arrangement of the orifice plate, ink barrier layer, and thin film substructure is illustrated at page 44 of the Hewlett-Packard Journal of February 1994, cited above. Further examples of ink jet printheads are set forth in commonly assigned U.S. Pat. No. 4,719,477 and U.S. Pat. No. 5,317,346, both of which are incorporated herein by reference.
Considerations with thin film ink jet printheads include the need to insure that each of the heater resistors fires an ink drop when selected. Due to variation in the power dissipating parasitic resistance presented by the conductive traces leading between the heater resistors and power and ground contact pads, the ink firing signals provided to the heater resistors typically include a certain amount of over-energy. This means that some resistors ultimately receive more than enough energy to a fire an ink drop while others receive only enough energy to fire an ink drop. Excessive energy has various negative effects including reduced resistor life, xe2x80x9ckogationxe2x80x9d which is the accumulation of a ink components that are tenaciously adhered to the passivation layer in the ink chambers, and reduced printhead reliability. Also, application of different energies to different resistors results in inconsistent bubble nucleation and drop formation.
While trace width variation is a known technique for energy balancing, use of such technique makes it difficult to reduce the width of the thin film substructure of the printhead.
There is accordingly a need for an improved ink jet printhead wherein heater resistors are more uniformly energized.
The disclosed invention is directed to an ink jet printhead having heater resistor energizing FET drive circuits that are configured to compensate for variation in power trace parasitic resistances, so as to reduce the variation in the energy provided to the heater resistors of the printhead.